Nitride semiconductor devices utilize characteristics such as a high saturated electron velocity and a wide band gap and have been actively developed as high-voltage and high-output semiconductor devices. Regarding the nitride semiconductor devices, many reports on electric field effect transistors, in particular, high electron mobility transistors (HEMTs) have been published. In particular, an AlGaN/GaN.HEMT having an electron transit layer of GaN and an electron supply layer of AlGaN has drawn attention. In the AlGaN/GaN.HEMT, a strain occurs in AlGaN due to a difference between the lattice constants of GaN and AlGaN, and the piezoelectric polarization caused by this strain and the spontaneous polarization of AlGaN generate a high-density two-dimensional electron gas (2DEG). Consequently, a high-voltage and high-output can be realized.
Japanese Patent Laid-open Nos. 2009-76845 and 2005-244072 disclose technologies relating to the above.